/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 */

/*!
 * Based on 'gatsby-remark-autolink-headers'
 * Original Author: Kyle Mathews <mathews.kyle@gmail.com>
 * Copyright (c) 2015 Gatsbyjs
 */

const toString = require('mdast-util-to-string');
const visit = require('unist-util-visit');
const slugs = require('github-slugger')();

function patch(context, key, value) {
  if (!context[key]) {
    context[key] = value;
  }

  return context[key];
}

const svgIcon = `<svg aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg>`;

module.exports = (
  {markdownAST},
  {icon = svgIcon, className = `anchor`, maintainCase = false},
) => {
  slugs.reset();

  visit(markdownAST, 'heading', node => {
    // Support custom-id syntax.
    const rawHeader = toString(node);
    const match = /^.+(\s*\{#([a-z0-9\-_]+?)\}\s*)$/.exec(rawHeader);
    const id = match ? match[2] : slugs.slug(rawHeader, maintainCase);
    if (match) {
      // Remove the custom ID part from the text node.
      const lastNode = node.children[node.children.length - 1];
      lastNode.value = lastNode.value.replace(match[1], '');
    }

    const data = patch(node, 'data', {});

    patch(data, 'id', id);
    patch(data, 'htmlAttributes', {});
    patch(data, 'hProperties', {});
    patch(data.htmlAttributes, 'id', id);
    patch(data.hProperties, 'id', id);

    if (icon !== false) {
      node.children.unshift({
        type: 'link',
        url: `#${id}`,
        title: null,
        data: {
          hProperties: {
            'aria-hidden': true,
            class: className,
          },
          hChildren: [
            {
              type: 'raw',
              // The Octicon link icon is the default. But users can set their own icon via the "icon" option.
              value: icon,
            },
          ],
        },
      });
    }
  });

  return markdownAST;
};
